In an image forming apparatus, a magnetic brush type developing device is generally employed to develop a latent image formed on a photoconductor serving as an image bearer with two-component developer composed of toner and magnetic carrier, as described, for example, in Japanese Patent Application Laid Open No. 10-239914 (JP-H10-239914-A). In such a developing device, a hollow cylindrical developing drum that accommodates multiple magnetic poles is generally employed and is freely rotated in the image forming apparatus. The developing drum carries and conveys the developer to a developing region, in which the photoconductor bearing a latent image is opposed thereto, and develops the latent image with the toner.
In such a developing process, a phenomenon called an edge effect frequently occurs. Specifically, excess toner is attracted to a thin line, a dot having a small diameter, or an outline of a solid image or the like. Consequently, an image is rarely uniformly formed, because the thin line and the dot of small diameter are fattened, and accordingly a latent image is not highly precisely reproduced on the photoconductor.
It is known that such an edge effect can be suppressed when a resistance of the magnetic carrier is decreased and a dielectric constant thereof is increased. Further, use of a low-resistance magnetic carrier decreases electric resistance of the total developer while increasing intensity of a development electric field, thus developing performance can be improved at the same time.
In general, magnetic carrier is borne on a developing drum by a magnetic force. However, the magnetic carrier acquires an electric charge after an electrostatic induction or electric charge injection process, so that an electrostatic force is generated between the electric charge of the magnetic carrier and that on a photoconductor. In such a situation, the smaller the electric resistance of the magnetic carrier, the stronger the electrostatic force due to the electrostatic induction or electric charge injection. For this reason, when the electric resistance of the magnetic carrier is excessively low, the electrostatic force arising between the magnetic carrier and the photoconductor exceeds the magnetic force arising between the magnetic carrier and the developing drum, so that the magnetic carrier becomes readily attracted to the photoconductor.